1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus of the type suitable for obtaining medical diagnostic information relating to a patient.
2. Description of the Prior Art
PCT Application WO 91/19994 discloses a diagnostic magnetic resonance apparatus having an examination space that is suitable for the acceptance for at least a part of a patient, a radio-frequency antenna for the transmission of excitation pulses into the examination space and/or for the reception of magnetic resonance signals from the examination space, and a gradient coil system for generating gradient magnetic fields in the examination space. The radio-frequency antenna is arranged closer to the examination space than is the gradient coil system, and a radio-frequency shield is arranged between the radio-frequency antenna and the gradient coil system for decoupling purposes. This radio-frequency shield has a first, electrically conductive film or foil or layer arrangement (henceforth only the term "layer" will be used herein, which encompasses the terms "film and foil"), and a second electrically conductive layer arrangement lying opposite the first arrangement, these layer arrangements being separated from one another by a dielectric. The layer arrangements have interconnects arranged, side-by-side that are separated from one another by electrically insulating slots, with the slots in the first layer arrangement arranged offset relative to the slots in the second layer arrangement.
The radio-frequency shield in this known apparatus is fashioned such that it is transmissive for the signals in the low-frequency range generated by the gradient coil system but is impermeable for the signals in the high-frequency range generated by the antenna. Such a frequency behavior is achieved by the measures disclosed as well in European Application 0 151 726. The radio-frequency shield is slotted so that the eddy currents generated in the radio-frequency shield by the gradient coil system are slight. The slots, however, reduce the shielding property of the high-frequency signals emitted by the antenna. The radio-frequency shield therefore includes the aforementioned second layer arrangement that is likewise slotted and whose slots are offset compared to the slots in the first layer arrangement. The high-frequency current generated by the antenna can now flow in a circuit which is closed by means of the capacitances that are formed in this way. Due the dielectric losses in the radio-frequency shield, however, the Q of the antennas is lowered. In particular, the whole body antenna is affected as a result because of its spatial proximity to the radio-frequency shield.